The present invention relates to an antenna system using a helical antenna for the reception of radiofrequencies and especially for the reception of microwaves.
A helical antenna consists of a single conductor or multiple conductors wound into a helical shape. Beside some other possible modes a helical antenna is normally used in a so-called axial mode or in a normal mode. The axial mode provides maximum radiation along the helix axis, which occurs when the helix circumference is of the order of one wavelength. The normal mode which yields radiation broadside to the helix axis, occurs when the helix diameter is small with respect to a wavelength. For the application according to the present invention the axial mode is of special interest.
The use of helical antennas for such antenna systems are widely known. For example U.S. Pat. No. 3,184,747 presents a coaxial feed helical antenna which has a director disk between feed and helix producing endfire radiation towards the disk. In this U.S. Patent the dimensions of the helix for such an antenna system are given.
U.S. Pat. No. 4,742,359 presents an antenna system using a helical antenna with two ends where the first end is linked to a feeder line. For the purpose of the following explanation it is understood that the said feeder line is aligned with the axis of the said helical antenna. Such a helical antenna may be built as a so-called endfire helical antenna, where under maximum received power conditions the direction of the signal power flow at the said first end is in the same direction as the received radiation. Such a helical antenna can also be built as a so-called backfire helical antenna, where under maximum received power conditions the direction of the signal power flow at the said first end is in the opposite direction to the received radiation.
In said U.S. patent an antenna system is presented, which comprises a reflector, a primary helical antenna having a coil with a pair of ends, said coil located at the focal point of said reflector so that the axis of the helical antenna coincides essentially with the axis of said reflector. A feeder line couples the antenna system with an external circuit, so that said primary helical antenna represents a backfire helical antenna coupled with said feeder line at the nearer end from said reflector and the other end of the helical antenna is free standing, and said feeder line is a coaxial cable.
It is further known from the international publication WO 92/13373 to use one or more helical feeders together with a dielectric lens. Thereby signals from several directions can be received simultaneously.
In the axial mode a helix wound like a right-hand screw receives right-hand circular polarization, while a helix wound like a left-hand screw receives left-hand polarization. This means known systems for the reception of different circular polarizations have two or more helices. For the reception of linear polarized radiation known systems use two or more helices wound in opposite directions. These helices can be provided side by side or can be connected in series.
Such a known antenna system for the reception of different polarizations is quite bulky. When such feeders are used together with concentration means, e.g. such as a parabolic reflector, a dielectric lens or the like, the helical antenna, or more precisely its phase center, must be coincident with the focal point of the concentration means, for each sens of polarization. Using two separate helices is sometimes inacceptable in a point of view of gain degradation and/or mutual coupling between the two opposite polarized helices due to inevitable defocussing and/or proximity.
It is an object of the present invention to provide a compact antenna system, for receiving several electromagnetical, preferably microwave, signals with different polarizations.